feat: Add 'merge_pipelines_enabled' project attribute

Boolean. Enable or disable merge pipelines.

See:
https://docs.gitlab.com/ee/api/projects.html#edit-project
https://docs.gitlab.com/ee/ci/pipelines/merged_results_pipelines.html

Author: nickbroon

gitlab/v4/objects/projects.py

@@ -645,6 +645,7 @@ class ProjectManager(CRUDMixin, RESTManager):
             "jobs_enabled",
             "lfs_enabled",
             "merge_method",
+            "merge_pipelines_enabled",
             "merge_requests_access_level",
             "merge_requests_enabled",
             "mirror_trigger_builds",
@@ -709,6 +710,7 @@ class ProjectManager(CRUDMixin, RESTManager):
             "jobs_enabled",
             "lfs_enabled",
             "merge_method",
+            "merge_pipelines_enabled",
             "merge_requests_access_level",
             "merge_requests_enabled",
             "mirror_overwrites_diverged_branches",